1. Field of the Invention
This invention relates to a voltage non-linear device which has non-linear voltage-current characteristics and is thus adapted for use in surge absorption, voltage stability, overvoltage suppression or switching devices, to a method for fabricating the same and to metallo-organic pastes useful for the fabrication. The invention also to a liquid crystal display device of the active matrix type using the voltage non-linear device and to a method for fabricating the display device.
2. Description of the Related Art
A so-called varistor device is known as a voltage non-linear device which exhibits an abrupt variation in resistance relative to an applied voltage and whose voltage-current characteristic is non-linear. The resistor used in this type of voltage non-linear device generally makes use of ceramic sintered products.
The ceramic varistors are made by sintering, for example, a ZnO main ingredient with small amounts of additives such as Bi.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Sb.sub.2 O.sub.3, CoO, MnO and the like.
FIG. 12 schematically shows an arrangement of a varistor device which is a known voltage non-linear device using the sintered ceramics. In the figure, indicated at 61, 62 are electrodes, at 63 are particles each mainly composed of ZnO, at 64 is a intergranular layer, and at 65, 66 are leads for the electrodes.
As shown in the figure, the varistor device is constituted of the electrodes 61, 62 between which a layer of the semiconductor particles is provided. The semiconductor particles are obtained by sintering a primary component of ZnO with small amounts of additives such as Bi.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Sb.sub.2 O.sub.3, CoO, MnO and the like.
The varistor device makes use of the Schottky barrier which is formed by producing insulating layers such as of Bi.sub.2 O.sub.3 in the intergranular layer 64 along with the growth of semiconductor ZnO crystals 63 at the time of the sintering.
A thickness of the existing voltage non-linear devices produced by ceramic sintering process is usually several millimeters. This makes it very difficult to apply it to thin-layered electronic parts and devices such as IC circuits, LCD, LED, and other panel types of displays (hereinafter referred to as display panels).
In order to ensure application of the device to the above-indicated electronic parts and devices or to realize miniaturization and formation of thin films or devices, there has been studied formation of thin or thick voltage non-linear devices.
The voltage non-linear device produced by thick film process is more reduced in the costs of fabrication apparatus than those devices produced by known ceramic sintering process, and can be miniaturized.
In this case, however, since the crystal particles are large in size, a film thickness of several tens micrometers is necessary in a minimum, disenabling one to attain the purpose of applying to IC circuits or display panels. Accordingly, it will be necessary to develop voltage non-linear devices or thin film varistor devices which are applicable to IC circuits or display panels.
A thin film voltage non-linear device is set forth, for example in Journal of Applied Physics, Vol. 50, pp. 555-558 (1979), in which ZnO and Bi.sub.2 O.sub.3 layers are successively deposited on an insulating substrate with electrodes by sputtering, thereby imparting non-linear characteristics by use of the boundary between ZnO and Bi.sub.2 O.sub.3.
FIG. 13 is a sectional view illustrating another structural arrangement of a thin film varistor device for use as a voltage non-linear device. In the figure, indicated at 71 is an insulating substrate, at 72 is a bottom electrode, at 73 is a ZnO semiconductor layer, at 74 is a Bi.sub.2 O.sub.3 insulating layer, at 75 is a ZnO semiconductor layer, and at 76 is a top electrode.
This thin film varistor device is prepared by forming the bottom electrode 72 on the insulating substrate 71, on which the ZnO semiconductor layer 73, the Bi.sub.2 O.sub.3 insulating layer 74 and the ZnO semiconductor layer 75 are deposited in this order. Finally, the top electrode 76 is formed on the layer 75.
Unlike the device produced by a ceramic sintering process, the voltage non-linear device (varistor device) of the above-described type is imparted with non-linear characteristics at the boundary between the ZnO semiconductor layers 73, 75 and the Bi.sub.2 O.sub.3 insulating layer 74.
The prior art techniques relating to this type of voltage non-linear device are set forth, for example, in Japanese Laid-open Patent Application Nos. 2-258968 and 2-96301.
In recent years, liquid crystal display devices have been widely in use as a display device.
Liquid crystal display devices are fundamentally of the light-receiving type and are ones which are low in both voltage and power. The arrangement of the device is simple and can be driven with IC, so that the liquid crystal display devices can be provided as a small-size and thin display means, thus being predominantly employed in the fields of display devices such as of electronic portable calculators and wrist watches.
Moreover, as information processors such as word processors, personal computers and the like have now come into wide use, there is a requirement for such processors being portable. In order to make thin, small-size processors which satisfy the above requirement, liquid crystal display devices have now been adopted in place of currently employed CRT.
For use as display means such as of word processors and personal computers, a significantly greater number of picture elements than those for electronic portable calculators or wrist watches are necessary so as to display characters or other letters and figures. If a so-called simple matrix system having electrodes crossed in X-Y forms is used for driving, the electrodes for driving individual picture elements are not independent from one another. As a result, a given voltage is applied to adjacent picture elements, not ensuring a complete non-display condition. Thus, so-called crosstalk takes place, resulting in a lowering of display quality.
In order to overcome the crosstalk deficiency, it is necessary to provide non-linear devices, such as diodes, thin film transistors and the like, for every picture element. However, this presents the problem that it is difficult to prepare non-linear devices, such as diodes or thin film transistors, for all picture elements amounting to several thousands to several hundred thousands in number, without deficiencies as having similar characteristics over a large area.
In order to solve the problem, as set forth in IEEE TRANSACTION ON ELECTRON DEVICES, Vol. ED-26, No. 8, August, 1979, pp. 1123-1128, there has been proposed a liquid crystal device which is driven under time division by use of varistor devices.
The liquid crystal display element makes use of two insulating substrates sandwiching a liquid crystal therebetween, one of the substrates being a varistor substrate. Thus, it is not possible to make a transmission-type liquid crystal display device.
To overcome the above defect, Japanese Laid-open Patent Application No. 2-291528 proposes a structure which comprises pixel electrodes signal lines for transmitting signals to the electrodes, and varistors connected with those electrodes in a planar structure on an insulating substrate. The varistors are produced by a thick film process which comprises printing and sintering process.
The voltage non-linear device known as the varistor is generally employed as a surge absorber. As shown in the voltage-current characteristic of FIG. 11, the resistance is great at a level smaller than a certain voltage (called varistor threshold voltage, Va) under which substantially little current passes. Over the voltage Va, the resistance is abruptly reduced with current I being passed. This is true of the case using a negative voltage, -Va.
As set forth hereinbefore, such a varistor device (voltage non-linear device) makes use of the Schottky effect which is produced by sintering a Z.sub.n O-based ceramics with small amounts of additives such as Bi.sub.2 O.sub.3, SiO.sub.2, PbO, CoO and MnO.
The varistor device can be arbitrarily controlled in varistor voltage, current capacitance and the like by controlling the distance between the electrodes and the size of particles and has been applied to various fields such as for protection of electronic circuits and of lightning rods.
However, with the known voltage non-linear device (thin film varistor device), a plurality of built-up structures consisting of the bottom electrode 72, the ZnO semiconductors 73, 75, the Bi.sub.2 O.sub.3 insulating layer 74 and the top electrode 75 shown in FIG. 13 have to be formed according to a fabrication process using a vacuum apparatus. This undesirably requires a number of steps with difficulties being involved in the manner of passing a plurality of gases and the control of pressure. Thus, there arises the problem that it is difficult to keep uniform film thickness and quality over a large area. In addition, installation costs and running costs of film-forming apparatus such as a sputtering apparatus and vacuum deposition apparatus are high, with the attendant problem that the resultant thin films become high in costs.
Liquid crystal display devices using the voltage non-linear device as a driving device have the following problems.
FIG. 14 is a sectional view schematically showing a liquid crystal display device using a voltage non-linear device as a drive device. Indicated at 1a is a glass substrate, at 1b is a counter glass substrate, at 2a is a pixel electrode, at 3a is a varistor device (voltage non-linear device), at 4a is a signal line transmitting signals to the pixel electrode, at 5a is a segment electrode and at 6a is a liquid crystal.
FIG. 15a to 15c are, respectively, an illustrative view of an essential part of FIG. 14, in which FIG. 15a is a sectional view showing an electrode arrangement at the lower glass substrate, FIG. 15b is a sectional view showing one pixel at the electrode arrangement, and FIG. 15c is a plan view of one pixel electrode at the electrode arrangement.
In this liquid crystal display device, the voltage non-linear device is formed by applying a paste for thick film varistor (i.e. a paste for a thick film voltage non-linear device) according to a printing technique and sintering the applied paste. This formation process is easier and lower in cost than thin film formation techniques such as sputtering, CVD and the like, enabling one to form a great number of devices at one time over a wide area.
However, the shape in section of the varistor device 3a formed by the printing technique is a kind of protuberance, as shown in FIG. 15b, and its control in thickness is not easy. When formed by the printing technique, the resultant film has a minimum thickness of not smaller than 10 .mu.m with a scattering of the film thickness being approximately within .+-.3 .mu.m.
Accordingly, the thickness of the liquid crystal layer 6a has to be 13 .mu.m in a minimum, thereby causing the contrast of the liquid crystal to be lowered.
When the varistor paste is subjected to screen printing, the paste is run, so that the extension from an opening of the screen at opposite sides ranges from 40 to 50 .mu.m.
The thus formed thick film varistor 3a is unlikely to undergo fine processings such as etching. Thus, fine, precise and uniform formation in shape of the varistor 3a according to the printing technique is very difficult, thus presenting the problem that the printing technique is not suitable for application to full color liquid crystal display devices wherein the pitches of pixels are in high density.
The thick film varistor having such a planar structure as shown in FIGS. 14 and 15 is disadvantageous in that since the threshold voltage of the varistor is determined depending on the space between the electrode 2a and the signal line 4a and the particle size of the varistor paste, the threshold voltage becomes high with a great scattering, resulting in a high drive voltage of the liquid crystal display device.